The composition distribution of a polyolefin such as an ethylene alpha-olefin copolymer refers to the distribution of comonomer (short chain branches) among the molecules that comprise the polyethylene polymer. When the amount of short chain branches varies among the polyethylene molecules, i.e., the amount of comonomer per 1000 carbons atoms varies with the length of the polyethylene molecules, the resin is said to have a “broad” composition distribution. When the amount of comonomer per 1000 carbons is similar among the polyethylene molecules of different chain lengths, the composition distribution is said to be “narrow.”
The composition distribution is known to influence the properties of copolymers, for example, extractables content, environmental stress crack resistance, heat sealing, and tear strength. The composition distribution of a polyolefin may be readily measured by methods known in the art, for example, temperature raising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF).
Polyolefins such as ethylene alpha-olefin copolymers are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization takes place in the presence of catalyst systems such as those employing, for example, a Ziegler-Natta catalyst, a chromium based catalyst, a metallocene catalyst, or combinations thereof.
It is generally known in the art that a polyolefin's composition distribution is largely dictated by the type of catalyst used and typically invariable for a given catalyst system. Ziegler-Natta catalysts and chromium based catalysts produce resins with broad composition distributions, whereas metallocene catalysts normally produce resins with narrow composition distributions. However, U.S. Pat. No. 6,242,545 and WO 2004/000919 disclose certain metallocenes, such as hafnocenes, that produce polyethylenes having a broad composition distribution.
Although the composition distribution is primarily dictated by the catalyst system used, attempts have been made to change the composition distribution of a polyolefin. For example, a desired composition distribution may be achieved with polymer blends. U.S. Pat. No. 5,382,630 discloses, inter alia, linear ethylene interpolymer blends made from components that can have the same molecular weight but different comonomer contents, or the same comonomer contents but different molecular weights, or comonomer contents that increase with molecular weight.
Another way to change the composition distribution utilizes multiple catalysts that respond differently to the comonomer concentration present in the reactor as is disclosed in, for example, U.S. Patent Application Publication Nos. 2004/0225088 and 2004/0122054.
And still other ways to produce polyolefins having desired composition distributions is through the use of multiple reactors with one or more catalyst systems and/or with the use of a condensable agent in the reactor. For example, W02006/007046 discloses, inter alia, a method of broadening the composition distribution breadth index (CDBI) of a single reactor/single catalyst system by increasing the amount of condensable agent in the reactor. However, sometimes there is no condensable agent present in the reactor or increasing the amount of condensable agent is not feasible because doing so would introduce particle stickiness and/or operability problems.
Other background references include WO 01/49751, WO 01/98409, EP 1 669 373 A, and U.S. Patent Application Publication Nos. 2004/121922 and 2005/148742.
Thus, methods to control the composition distribution of a polyolefin, such as an ethylene alpha-olefin copolymer, without having to use mixed catalysts, multiple reactors, condensable agents, and/or post reactor blending would be desirable and advantageous.